The purpose of the proposed research is to investigate the relationship of dose and plasma level of haloperidol to learning and maladaptive behavior in mentally retarded young adults. The goal is to develop a method for determining a dosage level of neuroleptic medication which maximally reduces problem behavior while minimally altering learning ability. Three dosage levels of haloperidol and placebo will be administered in increasing and decreasing dosages to 20 mentally retarded adults who have had a history of psychoactive drug mediction. Dosages will be gradually increased or decreased to target levels over one week and maintained at that level for three weeks. Effects of haloperidol dosage upon several variables will be investigated. Learning will be measured using a repeated acquisition technique in which the subjects learn a new, but similar, responses sequence each day. Maladaptive behavioral effects will be analyzed through daily ratings of target problem behaviors on a Maladaptive Behavior Scale and direct frequency recordings. Scores on each subscale of the Adaptive Behavior Scale, which will be administered pre-intervention and at placebo, will be analyzed also. Adverse side effects will be measured weekly through use of the Dyskinesia Identification System. The Peabody Picture Vocabulary Test, given pre-intervention and at placebo, will measure effects of neuroleptic dosage on intellectual functioning. Blood, drawn twice in the final week under each dosage level, will be analyzed to correlate plasma levels of haloperidol with dosage levels as well as to the learning and behavioral data. This research should provide a useful model for evaluating clinical efficacy and adverse effects of this neuroleptic drug as it relates to levels of such agents in biological fluids.